The study tested 254 women of childbearing age who reported eating as much or slightly more fish than the government recommends for pregnant women (8 to 12 ounces per week). Nearly three in 10 women were found to have more mercury in their bodies than the Environmental Protection Agency says is safe—and a level many experts say is far too high for pregnant women.

And although the women in the study ate more fish than the average American, nearly 60 percent still didn’t get the amount of omega-3s recommended during pregnancy from seafood in their diets, the study found.

“The advice from the [Food and Drug Administration] and [Environmental Protection Agency] should be more detailed and specific,”says Sonya Lunder, the study’s author and a senior analyst at EWG. “Women think they’re eating a healthy diet because they’re consuming a lot of fish, which is low in saturated fat and has lots of healthy minerals. But they don’t realize they’re at risk for high levels of mercury, too.”

The key, EWG reports, is not just watching how much fish you eat, but also the kinds of fish you consume.

EWG advises avoiding king mackerel, marlin, orange roughy, shark, swordfish, and tilefish, and limiting consumption of tuna in all forms.

Best bets? According to the EWG, it’s wild salmon, sardines, farmed rainbow trout, mackerel, oysters, mussels, and herring. “These fishier varieties are higher in fat and lower on the food chain, but they accumulate less mercury,” says Lunder.

Fish selection is also crucial when it comes to getting maximum omega-3s. “You can eat 20 servings of shrimp or just a serving and a half of salmon and get the same benefits,” Lunder says.

While the mercury-related concern is more imminent for women who are currently pregnant, all women should take EWG’s suggestions into consideration, particularly if they’re thinking about getting pregnant in the next year or so, Lunder says. “It takes a little while to make these dietary changes and to see the mercury levels drop,” she says. “It’s worth keeping an eye on high-mercury fish consumption for up to a year before getting pregnant.”

Wondering how your pescatarian-like habits stack up? EWG’s Good Seafood Guide and Seafood Calculator tools can help you find fish that’s high in omega-3 fatty acids and low in mercury.